Lhermitte-Duclos disease (LDD), also known as dysplastic gangliocytoma of the cerebellum, is a rare, usually benign, slow-growing tumor, that commonly affects patients aged 30 to 50 years-old. The manifestations of dysplastic cerebellar gangliocytoma are nonspecific and are related both to the mass effect produced by its growth and to the location of the lesion. Cerebellar symptoms such as ataxia are often present. In 40% of cases, the tumor is associated with Cowden syndrome, which is part of a group ofgenetic disorders called polypoid hamartoma complex. In this case report, the patient presented expansive lesion in the posterior fossa, compatible with LDD, associated with macrocephaly. These findings are consideredmajor criteria for Cowden syndrome. When together, they confirm the diagnoses. To our knowledge, this is the first report of the association of LDD and Cowden syndrome in Brazil.